1. Field of the Invention
The present invention relates to an image forming apparatus and a droplet ejection control method, and more particularly, to droplet ejection control technology for a liquid ejection apparatus which forms, on a medium, a shape such as an image or a prescribed pattern by ejecting a liquid droplet from a ejection hole.
2. Description of the Related Art
In recent years, inkjet printers have come to be used widely as data output apparatuses for outputting images, documents, or the like. By driving recording elements, such as nozzles (ejection holes), provided in a recording head in accordance with data, an inkjet printer is able to form data onto a recording medium by means of ink ejected from the nozzles. In an inkjet printer, a desired image is formed on a medium by ejecting ink droplets from nozzles in a head while the head and a recording medium are caused to move relatively to each other.
Inkjet printers have been used as apparatuses for outputting documents principally in homes and offices, and recently, they have started to be used for outputting images captured by digital cameras, and the like. Furthermore, there are also inkjet apparatuses in which A3 and poster size recording media can be used, and hence they have come to be used for outputting publicity prints, posters, or the like.
In the printing of images captured by means of a digital camera, or the like, good image resolution is an important requirement in image printing, and high-quality image printing is achieved by developments such as multi-color printing, multiple tone gradation, finer dot size, higher dot density, and the like. For example, by using multiple ink colors, such as light inks, it is possible to achieve full color and multiple-stage tone gradation. By increasing the density of the nozzle arrangement and reducing the droplet size, it is possible to increase dot density and reduce dot size in the image. Moreover, if droplet ejection control is performed in order that ink is ejected in such a manner that adjacent dots are mutually overlapping, then the dots can be formed to a high density on the media.
However, when adjacent dots are formed in a mutually overlapping fashion, if the subsequent ink droplet lands before the previously ejected ink has become fixed on the media, then the shape of the dots can be disrupted, and consequently, the subsequently deposited ink droplet can move toward the previously deposited ink droplet. As a result, image abnormalities, such as streaking and non-uniformity, can occur in the image. Furthermore, if inks of different colors are deposited in an overlapping fashion, then color mixing can occur, and consequently it becomes difficult to achieve the desired color and tonal gradation.
In general, in order to prevent image abnormalities caused by streaking or landing interference, a method has been proposed in which droplet ejection is controlled in such a manner that a subsequent ink droplet is ejected after a previously deposited ink droplet has permeated to a certain degree, or a method has been proposed in which a temperature adjustment device is provided for warming a media on which ink has been deposited or warming the ink deposited on a media, the fixing of the ink being accelerated by means of this temperature adjustment device. Furthermore, a method has also been proposed in which an ultraviolet-curable ink whose curing can be promoted by the irradiation of ultraviolet light is used as the ink for forming an image, and the fixing of ink deposited onto a media is promoted accordingly.
Japanese Patent Application Publication No. 6-183129 discloses an inkjet recording method and an inkjet recording apparatus based on an inkjet recording method whereby a plurality of recording heads disposed in a parallel arrangement are moved relatively with respect to a recording medium in order to perform recording. According to the inkjet recording method and inkjet recording apparatus, recording timings are staggered between the recording of either one of the ink dots constituting a border between ink dots of different inks, and the recording of the other ink dot(s), in such a manner that landing interference between different colors is prevented.
Furthermore, Japanese Patent Application Publication No. 2002-120361 discloses the inkjet recording apparatus which comprises a drum for fixing a sheet of paper in position and a plurality of inkjet heads disposed facing the drum at prescribed intervals apart in the circumferential direction of the drum, color printing being performed onto the sheet of paper by driving the inkjet heads while the drum is rotated. In the inkjet recording apparatus, the time T until dots of different colors make contact or overlap mutually at their landing points on the paper satisfies the following relationship, T≧10 msec, in such a manner that bleeding is prevented.
Furthermore, Japanese Patent Application Publication No. 2000-177115 discloses a printing method and a print head apparatus in which a charged ink is used, and a channel for ejecting ink is provided between electrodes which generate an electric field. The electric field acts on the ink ejected from the channel and thus deflects the direction of ejection of the ink, in such a manner that image degradation caused by non-uniformities is prevented.
Moreover, Japanese Patent Application Publication No. 2000-185403 discloses an inkjet nozzle, an inkjet recording head, an inkjet cartridge and an inkjet recording apparatus, in which a plurality of heaters that generate air bubbles in ink are provided with respect to each nozzle. By controlling the heaters, different types of bubbles are generated in the ink and hence the direction of flight of the ink can be deflected, in such a manner that landing interference is prevented.
According to the inventions described in Japanese Patent Application Publication No. 6-183129 and Japanese Patent Application Publication No. 2002-120361, high image quality is achieved by preventing bleeding or decline in density, by restricting the landing timings between different colors; however, landing interference between inks of the same color are unresolved, and furthermore there is no improvement in high-speed printing. Furthermore, the inventions described in Japanese Patent Application Publication No. 2000-177115 and Japanese Patent Application Publication No. 2000-185403 relate to technologies for deflecting the direction of flight of ejected ink droplets and thereby preventing image degradation caused by non-uniformities; however, these publications do not disclose or suggest a control method for preventing landing interference or obstacles to such a control method.